Slugify: Lowercase and Umlaut support

I suggest the following improvements:

1. convert UPPER to lower case (or have this at least as an option to be backward compatible and/or for use cases I do not know)

2. support German Umlauts, i.e. ä|ö|ü → ae|oe|ue. currently the function does ä|ö|ü → a|o|u . Example: URL of the city of Munich, which is called München in German, is

   www.muenchen.de, not www.munchen.de, so the correct slug of München is muenchen